As set out for example in document FR 2 764 544 by the Applicant, the stretch rod is conventionally introduced into the opening (the neck) of the parison which is to be stretched and blown. This stretch rod slides through a member known as a nozzle. The rod slides axially along the longitudinal axis of the nozzle and an annular space is left unoccupied around this rod for the blowing fluid to pass through.
Stretching a PET parison and preblowing it at medium pressure (e.g. 7 bar) lead to an axial elongation speed of around 0.8 to 1.2 m/s. The radial stretching produced by high-pressure blowing (e.g. 40 bar) presses the material against the mold walls which are cooled, for example by an internal chilled water circuit.
It is very important for the quality of the blow-molded product to control the speed of axial elongation. It is also important to synchronize the movement of the stretch rod with the start of blowing. The kinematics of the stretch rods must therefore be controlled, but control is difficult, especially in modern stretch-blow molding machines which run at very high speeds.
The stretch speed is conventionally controlled by a roller and cam device mounted at the top of the machine. This device is so conventional that it is often not even depicted (see for example the Applicant's document FR 2 863 929).
This roller and cam device is designed to synchronize the axial position of the stretch rod with the angular position of the blow molding station around the axis of rotation of the carousel.
The appended FIG. 1, which is taken from the Applicant's document FR 2 814 392, illustrates a conventional configuration of this roller and cam control.
The prior-art machine illustrated in FIG. 1 is rotary: it comprises several stretch-blow molding stations 12 mounted on a carousel 14. The carousel 14 is rotated continuously about its axis A1. Each stretch-blow molding station 12 comprises a blowing mold 16, a blowing nozzle 18, a stretch rod 20, and control means 22 for controlling the movements of the stretch rod 20. The rods 20 are attached at their top end to a slider 24 which can slide vertically on a rail (not shown) mounted on the carousel 14. The blow molding station comprises a pneumatic thrust cylinder controlled on a fully-on or fully-off basis and acting on the slider 24 to push it axially down. The stretching speed is controlled by a control cam that forms a helix about the axis A1 of the carousel 14. The slider 24 comprises a roller 38 which, under the action of the thrust cylinder 34, pushes against said cam. When the rod 20 rises, the control chamber of the thrust cylinder 34 is vented to atmosphere. At this point, because of the presence of the seal 40, only the lower end of the rod 20 is exposed to the blow molding pressure. The rod 20 is therefore now subjected to an upward axial force equal to the blow molding pressure multiplied by the transverse area of the rod in section. Depending on the individual case, this force may be of around 50 to 150 daN, which is enough to compensate for the weight of the rod 20 and of its control mechanism, and thus move the rod 20 back to its retracted position. It is preferable to provide a safety cam 42 similar to the control cam but designed to force the rod 20 back to its retracted position if the roller 38 touches it. The safety cam 42 ensures that the rod 20 is disengaged from the mold 16 when it is wished to eject the container at the end of the molding operation even if, owing for example to the preform 10 bursting during molding, the pressurized air escapes from the mold.
Document FR 2 863 928, also in the Applicant's name, sets out another form of stretch rod movement control, using a roller and control cam mechanism. Two fixed circular and superposed control cams each define on their inner face a rolling surface for two pusher rollers. These rolling surfaces are provided with humps. The pusher is made to rotate as the rollers roll over these humps. A compass mechanism converts the rotational movement of the pusher into a vertical linear reciprocating movement of the stretch rod.
The stretch rod movement control means described in document FR 2 814 392 or FR 2 863 928 are perfectly satisfactory in terms of the operation of the stretch-blow molding process.
However, the need to significantly reduce the height of stretch-blow molding machines has become clear. The arrangements described in documents FR 2 814 392 and FR 2 863 928 for the control cam and the control thrust cylinders make the machine very tall.
Furthermore, the presence of control cams near the mouths of the bottles as they come out of the molds can lead to accidental contamination because the cams are necessarily lubricated.
It is one of the objects of the invention to solve these problems.